1. Field of the Invention
The invention relates to an exhaust gas purification device for a compression-ignition combustion engine.
2. Description of the Related Art
An exhaust gas discharged from a compression-ignition combustion engine includes carbon particulates. The particulates discharged to the outside air may lead to environmental pollution.
Japanese unexamined patent publication (Kokai) No. 5-69311 discloses an exhaust gas purification device of a compression-ignition combustion engine, comprising exhaust branch passages each being connected to a corresponding cylinder of the engine at one end thereof and connected to a common exhaust passage at the other end thereof, and filters arranged in the exhaust branch passages to trap the carbon particulates, respectively.
At the first stage of trapping the carbon particulates, first thin carbon particulate layers are formed in the filters when the exhaust gas discharged from the engine flows into the filters. At the second stage of trapping the carbon particulates, the filters trap much particulates since the first carbon particulate layers facilitate trapping of the carbon particulates.
By the way, in a four-cylinder engine, an exhaust cycle in each cylinder is sequentially performed at every crank angle of 180.degree. (referring to FIG. 10). Therefore, when one of the cylinders is in the exhaust cycle, other cylinders are not in the exhaust cycle. As the exhaust gas discharged from the one cylinder flows into the exhaust passage of the exhaust manifold, the pressure the level in the common exhaust passage increases to a pressure level which is greater than atmospheric pressure. On the other hand, the pressure levels in the exhaust branch passages upstream of the corresponding filters, which passages are connected to the cylinders which are not in the exhaust cycle, are generally at atmospheric pressure.
Therefore, the exhaust gas discharged from the one cylinder flows back into the filters arranged in the exhaust branch passages which are connected to the cylinders which are not in the exhaust cycle. Due to the flow back of the exhaust gas into the filters, the first carbon particulate layers are removed from the filters. Thus, the capability of the filters for trapping the carbon particulates decreases.